1. Field of the Invention
The present invention relates to methods and means for detecting certain phase discontinuities in elongated liquid containment vessels or pipes, and more particularly to a new and novel technique and attendant apparatus for the effecting of same, which technique, and method thereof has been proven to be highly effective and accurate for detecting voids such as gas bubbles or, for that matter, complete gas blockages in liquid containment means.
It will be appreciated by those skilled in the art that the detection of gas bubbles or gas pockets within a liquid is of substantial importance in the effective and proper monitoring and controlling of a wide variety of processes. For example, transmission of pressure through liquid-filled sensing lines is affected by air within the lines. Air that completely fills the cross section of a sensing line can cause an error in transmission of static, or mean, pressure proportional to the vertical height of the air void. In addition, both small air bubbles and large air voids change the natural frequencies for pressure wave propagation in a sensing line (the standing wave frequencies) and, consequently, affect transmission of dynamic pressure fluctuations through the line. If the lowest natural frequency is reduced, by air, into the sensitive range of the pressure transmitter connected to a line, the transmitter signal can becomes significantly degraded by oscillations about the mean value of pressure.
As another example, in the operation of high temperature processes wherein are utilized liquid coolants the presence of gas bubbles within the coolant liquid can cause the heat transfer rate to be reduced wherever the bubbles are located, resulting in a lower overall heat transfer rate.
2. Description of the Prior Art
Numerous prior investigators have discovered, taught, and disclosed methods and means for detecting and monitoring the presence of discontinuities or inhomogeneities in fluids, such as for example gas bubbles in a liquid, and in particular in a liquid-filled pipe or other conduit.
The visual observation technique, employing an in situ sight glass or window, has been a rather popular innovation for detecting bubbles in liquid-filled pipes and one version has been familiar to a large section of the general population, at least up until the last several years, in the form of means for inspecting and determining the refrigerant sufficiency of many automotive air conditioners.
Liebermann, in U.S. Pat. No. 3,046,780, July 31, 1962, discloses a fluid condition monitor comprising a towable acoustic resonator and a pair of coupled transducers to detect impurities in the fluid through which it is towed by monitoring the quality factor, Q, of the resonating system. While it is suggested that this device can be used in pipelines, it would clearly be preferable to utilize a detection device which need not be placed within the mainstream of flow of fluid in the pipe or conduit.
Nakamoto, et al., in U.S. Pat. No. 4,144,741, Mar. 20, 1979, teach the concept of detecting voids or bubbles in a flowing liquid medium by inserting a detector in the flowing medium. The detector consists of one exciting coil attached to an AC signal applied thereto and two detecting coils located near the exciting coil. The outputs from the detecting coils provide means for detecting voids in the fluid.
The following three prior art references also show means for detecting voids or bubbles in a flow line and they also utilize transducers attached to the walls of the fluid-conveying pipe for inducing acoustical waves through the pipe and detecting the output through a second transducer (Blanchard, U.S. Pat. No. 2,573,390, Oct. 30, 1951; Wonn, U.S. Pat. No. 4,130,010, Dec. 19, 1978; and Liebermann, U.S. Pat. No. 4,235,095, Nov. 25, 1980).
The following three prior art references teach means for utilizing ultrasonic signals to detect bubbles in liquids; however, they have the distinct disadvantage of employing the use of rather complicated and expensive electronics in both the transmitting and receiving circuits (Taylor, U.S. Pat. No. 4,014,206, Mar. 29, 1977; McKnight, U.S. Pat. No. 4,112,735, Sept. 12, 1978; and Stasz, et al., U.S. Pat. No. 4,122,713, Oct. 31, 1978).
The concept of exciting standing waves in a fluid flowing medium is shown by Heisig, et al., U.S. Pat. No. 3,283,562, Nov. 8, 1962. The procedure includes coupling a transducer to the outer wall of a pipe and inducing signals through the pipe and the medium flowing therein to excite the standing waves of the medium. The resultant vibrations are detected by a second transducer which passes the signal through a demodulator whereby a trace is provided which will show or give an observation of a bubble passing through the pipe section in the region of the transducers. The input transducer is pulsed in a specific frequency band to avoid buildup of standing waves, a free air resonant frequency of the input transducer will then be known. The transducer is then excited to cause standing waves in the cavity whereby the output of the output transducer will indicate passage of the bubbles in the fluid.
There is no suggestion in any of the above prior art references of the technique of the present invention for using measured shifts in standing wave frequencies to detect the presence of gas voids in liquid-filled conduits. Nor is there any suggestion in the prior art references of the technique of introducing a pressure transient into a liquid-filled conduit in order to excite standing waves in the conduit. More particularly, there is no suggestion of methods or means suitable for detecting discontinuities, including voids, in conduits such as the sensing lines used in flow measurements necessary to the operation of nuclear power plants. In the teachings of the prior art references, the modus operandi is the detection of bubbles in the fluid as the bubbles are directed past a particular region or area physically near the detector. In operation of the technique of the instant invention, however, the bubbles in liquid do not need to be directed to or past a transducer; i.e., they may be physically located quite far from the detection device and the liquid need not be flowing.